1. Field of the Invention
The present invention relates to liquid crystal display devices wherein a ferroelectric liquid crystal is used.
2. Description of the Prior Art
Liquid crystals include those of chiral smectic C phase (hereinafter referred to simply as "SmC* liquid crystals") having a helical structure and exhibiting ferroelectric properties.
When such a SmC* liquid crystal is injected into a cell having a small gap, the helical structure of the SmC* liquid crystal is unwound under the influence of the interface, resulting in the conjoint presence of a domain wherein the liquid crystal molecules are stabilized as inclined at a tilt angle .theta. with respect to a normal of the smectic layer, and a domain wherein the liquid crystal molecules are stabilized as tilted at -.theta. in the opposite direction. When a voltage is applied to the SmC* liquid crystal within the cell, the liquid crystal molecules and the spontaneous polarization thereof can be oriented uniformly in a direction. When the polarity of the voltage to be applied is changed, the orientation of the liquid crystal molecules can be changed from a definite state to another state for switching drive. Switching drive causes the SmC* liquid crystals within the cell to produce altered birefringent light, so that the transmitted light is controllable by interposing the cell between two polarizers. Furthermore, when the application of voltage is discontinued, the orientation of the liquid crystal molecules is maintained in the state before the discontinuation by the ability of the interface to regulate the orientation. Thus, a memory effect is also available.
Accordingly, it has been attempted to utilize the memory effect of SmC* liquid crystals for providing high-duty liquid crystal display devices resorting to a multiplex drive system.
Such a liquid crystal display device comprises two glass substrates opposed to each other in parallel, and a plurality of transparent electrodes arranged in the form of parallel stripes and formed on each of the opposed surfaces of the substrates. The transparent electrodes on one of the glass substrates are arranged perpendicular to those of the other glass substrate. The electrode bearing surface of each substrate is further formed with an orientation film with an insulation film interposed therebetween. A layer of SmC* liquid crystals is provided between the two glass substrates.
The liquid crystal device has the following problem. The uneven presence of charges resulting from the spontaneous polarization of liquid crystal molecules produces an electric field (polarization electric field) within the device, impairing the memory effect in memory states, i.e., in a state wherein the orientation angle of the molecules is +.theta., and in a state wherein the angle is -.theta. (i.e., the memory effect in a bistable state).
Stated more specifically, a polarization electric field due to uneven presence of polarized charges of liquid crystal molecules is present within the cell even in the memory state as shown in FIG. 2 (a). Accordingly, in response to the polarization electric field, the liquid crystal molecules are altered in orientation so as to minimize the energy within the cell as seen in FIG. 2 (b). Presumably for this reason, it becomes impossible to assume a bistable memory state. Indicated at 6 in FIG. 2 is the C-director of liquid crystal molecules, and at 7 the spontaneous polarization of liquid crystal molecules. The tip of this arrow represents +pole, and the other end of the arrow represents - pole. Represented by 8 and 9 are positive and negative polarized charges, respectively, as unevenly present. Indicated at 10 is the presence of the polarization electric field in the direction of the arrow.
Accordingly, an object of the present invention is to provide a liquid crystal display device having a bistable memory effect prevented from impairment due to a polarization electric field.